[PATCH v2] hw/core: Allow setting 'virtio-blk-device.scsi' property on OSX host

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Commit ed65fd1a2750 ("virtio-blk: switch off scsi-passthrough by
default") changed the default value of the 'scsi' property of
virtio-blk, which is only available on Linux hosts. It also added
an unconditional compat entry for 2.4 or earlier machines.

Trying to set this property on a pre-2.5 machine on OSX, we get:

   Unexpected error in object_property_find() at qom/object.c:1201:
   qemu-system-x86_64: -device virtio-blk-pci,id=scsi0,drive=drive0: can't apply global virtio-blk-device.scsi=true: Property '.scsi' not found

Fix this error by marking the property optional.

Fixes: ed65fd1a27 ("virtio-blk: switch off scsi-passthrough by default")
Suggested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v2: Reworded description (Cornelia)

Extracted from testing series:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg675074.html
---
 hw/core/machine.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 3e288bfceb..d8e30e4895 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -148,7 +148,8 @@ GlobalProperty hw_compat_2_5[] = {
 const size_t hw_compat_2_5_len = G_N_ELEMENTS(hw_compat_2_5);
 
 GlobalProperty hw_compat_2_4[] = {
-    { "virtio-blk-device", "scsi", "true" },
+    /* Optional because the 'scsi' property is Linux-only */
+    { "virtio-blk-device", "scsi", "true", .optional = true },
     { "e1000", "extra_mac_registers", "off" },
     { "virtio-pci", "x-disable-pcie", "on" },
     { "virtio-pci", "migrate-extra", "off" },
-- 
2.21.1